I read some older posts on your blog about dispersal of lactase persistance world wide. Is it not so that everyone can digest lactose at birth and that the production of the enzyme lactase persists as long as milk consumption persists, whether it is human, goat or cows milk? There is also the matter of pasteurization which kills beneficial bacteria that help digest the lactose. Raw milk is better tolerated than pasteurized milk by all populations wordwide, and as far as I can tell; lactose intolerance is actually an intolerance to pasteurized milk.
Even those who drink pasteurized milk have plenty of beneficial bacteria, but bacterial digestion of lactase in the gut is problematic. The bacteria generate lactic acid and CO2, which in large quantities lead to malabsorption of other nutrients and discomfort.
Malabsorption apparently was not a barrier to early dairying peoples; today pastoralists who rely on milk but do not have lactase persistence tend to ferment or culture the milk in ways that cut lactose content.
The target of selection on lactase persistence was likely energy recovery. Lactose accounts for roughly 30 percent of the calories in milk, and increasing the fraction absorbed was probably highly beneficial, particularly for pregnant and lactating women. But some scientists think that the target of selection was the intestinal effect of lactose-absorbing bacteria, as diarrheal diseases exerted a high mortality risk in preindustrial peoples.
Small amounts of milk will not hurt anyone unless they have a milk allergy, which is a separate issue. Lactase production is universal in infants, children’s lactase production declines at an age that varies but is usually late childhood or adolescence.